1. Field of the Invention
The invention relates to a coding/decoding apparatus and, more particularly, to an apparatus for variable length coding image data after it has been orthogonal transformed on a block unit basis comprising a predetermined number of pixels and to a decoding apparatus for decoding the coding data which was coded by the coding apparatus.
2. Related Background Art
Generally, since the amount of information for an image signal is very large, in the case of recording the image signal by a digital recording and reproducing apparatus such as a digital VTR or the like, the amount of information is reduced by a highly efficient compression coding and the reduced image signal is recorded.
An orthogonal transformation coding using a DCT (Discrete Cosine Transform) is known as an example of the highly efficient compression coding techniques.
According to the orthogonal transformation coding, an input image signal is separated into blocks and each of the frequency components obtained by frequency dividing the image signal on such a block unit basis is coded. By using a method such that the amount of data which is allocated is reduced or the like with respect to the high frequency component whose influence on the deterioration of visual characteristics is small, the amount of information of the input image signal can be reduced.
The case where the signal which was compression coded by the above method is recorded and reproduced by the digital VTR will now be described.
When the coded data is recorded onto a tape, it is separated into sync blocks of a predetermined data amount unit as a minimum synchronous unit in the reproducing mode and the data is recorded.
FIG. 1 shows a construction of a sync block.
In FIG. 1, SYNC 1 denotes a synchronization pattern, ID 2 indicates identification information comprising address information or the like to indicate the position on the screen of the data recorded, and DATA 3 denotes image data which was compression coded.
FIG. 2 shows an arrangement on the tape of the sync blocks recorded by a conventional VTR.
Reference numerals 2a, 2b, and 2c in FIG. 2 denote tracks. The sync blocks are arranged in those tracks.
Each track is a locus of a recording head in the recording mode. In the normal reproducing mode, a reproducing head scans on the same locus as the locus which was scanned by the recording head in the recording mode, so that the image data in each sync block can be reproduced.
A method of reproducing the recording data in a special reproducing mode will now be described.
In FIG. 2, reference numeral 10 denotes a locus of the reproducing head in a high speed searching mode, one of the special reproducing modes.
Since a tape travelling speed in the high speed searching mode is faster than that in the normal reproducing mode, the locus 10 of the reproducing head is a locus such that the reproducing head scans a plurality of tracks.
Therefore, the data of only those sync blocks shown by hatched regions in FIG. 2 can be extracted from the tape by the reproducing head.
In the above conventional recording/reproducing method of image data mentioned above, however, a sync block in which only a part can be scanned exists in the reproducing head locus 10.
For example, there is a sync block like a sync block 11 in FIG. 2 such that the whole portion cannot be scanned although the front portion of the sync block can be scanned. Therefore, since the data arranged in the rear portion in the sync block 11 cannot be fetched, there is a problem such that the data which is necessary for the expanding process cannot be prepared and the image data existing in the sync block 11 cannot be reproduced.
In the case where a transmission error exceeding an error correction ability occurs on a transmission path, there is also a problem such that the data after the position at which the transmission error occurred cannot be reproduced and the data of a fixed length unit disappears in the worst case.
Under such circumstances, it is an object of the invention to provide a coding/decoding apparatus which can reproduce a wider region on a screen than that of a conventional apparatus in the case of the occurrence of a transmission error or in a special reproducing mode and which can minimize the deterioration of picture quality.
According to one preferred embodiment, the above object is accomplished by a coding apparatus comprising: block forming means for dividing an input 20 image signal into blocks; coding means for compression coding the block data obtained by dividing by the block forming means by using an orthogonal transformation; and generating means for generating (n) sync blocks from the coding data of (m) blocks, wherein each sync block has a first area in which important data for the orthogonally transformed coded block data for a special number of blocks is arranged and a second area in which non-important data for the orthogonally transformed coded block data for an unspecified number of blocks is arranged.
According to another preferred embodiment, there is provided a decoding apparatus in which an image signal is divided into blocks and the block-formed block data is compression coded and the compression coded block data is decoded on an (m) unit basis (m is a natural number), wherein the decoding apparatus comprises: first decoding means for decoding the coding data of the (m) inputted blocks on a block unit basis in accordance with the inputting order; second decoding means for decoding the coding data of the (m) inputted blocks on a block unit basis in accordance with the reversed inputting order; and output means for integrating -the data decoded by the first and second decoding means and for outputting the decoding data of the (m) blocks.
The above and other objects and features of the invention will become apparent from the following detailed description taken in conjunction with the appended claims with reference to the accompanying drawings.